sourcebuild.texi (Test directives): Document that arguments include-opts and exclude-opts are now optional for...

	* doc/sourcebuild.texi (Test directives): Document that arguments
	include-opts and exclude-opts are now optional for dg-skip-if,
	dg-xfail-if, dg-xfail-run-if, and dg-shouldfail.

	* lib/target-supports-dg.exp (check-flags): Provide defaults for
	include-opts and exclude-opts; skip checking the flags if arguments
	are the same as the defaults.
	(dg-xfail-if): Verify the number of arguments, supply defaults
	for unspecified optional arguments.
	(dg-skip-if, dg-xfail-run-if): Verify the number of arguments.

From-SVN: r157206